Integrin superfamily proteins are heterodimeric cell surface receptors, composed of an alpha and beta subunit. 18 alpha and 8 beta subunits have been reported, which have been demonstrated to form 24 distinct alpha/beta heterodimers. Each chain comprises a large extracellular domain (>640 amino acids for the beta subunit, >940 amino acids for the alpha subunit), with a transmembrane spanning region of around 20 amino acids per chain, and generally a short cytoplasmic tail of 30-50 amino acids per chain. Different integrins have been shown to participate in a plethora of cellular biologies, including cell adhesion to the extracellular matrix, cell-cell interactions, and effects on cell migration, proliferation, differentiation and survival (Barczyk et al, Cell and Tissue Research, 2010, 339, 269).
Integrin receptors interact with binding proteins via short protein-protein binding interfaces with ligands and the integrin family can be grouped into sub-families that share similar binding recognition motifs in such ligands. A major subfamily is the RGD-integrins, which recognise ligands that contain an RGD (Arginine-glycine-aspartic acid) motif within their protein sequence. There are 8 integrins in this sub-family, namely αvβ1, αvβ3, αvβ5, αvβ6, αv1β8, αIIbβ3, α5β1, α8β1, where nomenclature demonstrates that αvβ1, αvβ3, αvβ5, αvβ6, & αvβ8 share a common V subunit with a divergent β subunit, and αvβ1, α5β1 & α8β1 share a common β1 subunit with a divergent a subunit. The β1 subunit has been shown to pair with 11 different a subunits, of which only the 3 listed above commonly recognise the RGD peptide motif. (Humphries et al, Journal of Cell Science, 2006, 119, 3901).
Within the 8 RGD-binding integrins are different binding affinities and specificities for different RGD-containing ligands. Ligands include proteins such as fibronectin, vitronectin, osteopontin, and the latency associated peptides (LAPs) of Transforming growth factor β1 and β3 (TGFβ1 and TGFβ3). The binding to the LAPs of TGFβ1 and TGFβ3 has been shown in several systems to enable activation of the TGFβ1 and TGFβ3 biological activities, and subsequent TGFβ-driven biologies (Worthington et al, Trends in Biochemical Sciences, 2011, 36, 47). The specific binding of RGD integrins to such ligands depends on a number of factors, depending on the cell phenotype. The diversity of such ligands, coupled with expression patterns of RGD-binding integrins, generates multiple opportunities for disease intervention. Such diseases include fibrotic diseases (Margadant et al, EMBO reports, 2010, 11, 97), inflammatory disorders, cancer (Desgrosellier et al, Nature Reviews Cancer, 2010, 10, 9), restenosis, and other diseases with an angiogenic component (Weis et al, Cold Spring. Harb. Perspect. Med. 2011, 1, a006478).
A significant number of αv integrin antagonists (Goodman et al, Trends in Pharmacological Sciences, 2012, 33, 405) have been disclosed in the literature including antagonist antibodies, small peptides and compounds. For antibodies these include the pan-αv antagonist Intetumumab, the selective αvβ3 antagonist Etaracizumab, and the selective αvβ6 antagonist STX-100. Cilengitide is a cyclic peptide antagonist that inhibits both αvβ3 and αvβ5, and SB-267268 is an example of a compound (Wilkinson-Berka et al, Invest. Ophthalmol. Vis. Sci., 2006, 47, 1600), which inhibits both αvβ3 and αvβ5. Invention of compounds to act as antagonists of differing combinations of αv integrins enables novel agents to be generated and tailored for specific disease indications.
Pulmonary fibrosis represents the end stage of several interstitial lung diseases, including the idiopathic interstitial pneumonias, and is characterised by the excessive deposition of extracellular matrix within the pulmonary interstitium. Among the idiopathic interstitial pneumonias, idiopathic pulmonary fibrosis (IPF) represents the commonest and most fatal condition with a median survival of 3 to 5 years following diagnosis. Fibrosis in IPF is generally progressive, refractory to current pharmacological intervention and inexorably leads to respiratory failure due to obliteration of functional alveolar units. IPF affects approximately 500,000 people in the USA and Europe. This condition therefore represents a major unmet medical need for which novel therapeutic approaches are urgently required (Datta A et al, Novel therapeutic approaches for pulmonary fibrosis, British Journal of Pharmacology 2011 163: 141-172).
There are strong in vitro, experimental animal and IPF patient immunohistochemistry data to support a key role for the epithelial-restricted integrin, αvβ6, in the activation of TGF-β1. Expression of this integrin is low in normal epithelial tissues and is significantly up-regulated in injured and inflamed epithelia including the activated epithelium in IPF. Targeting this integrin therefore reduces the theoretical possibility of interfering with wider TGF-β homeostatic roles. Partial inhibition of the αvβ6 integrin by antibody blockade has been shown to prevent pulmonary fibrosis without exacerbating inflammation (Horan G S et al Partial inhibition of integrin αvβ6 prevents pulmonary fibrosis without exacerbating inflammation. Am J Respir Crit Care Med 2008 177: 56-65)
The αvβ3 integrin is expressed on a number of cell types including vascular endothelium where it has been characterised as a regulator of barrier resistance. Data in animal models of acute lung injury and sepsis have demonstrated a significant role for this integrin in vascular leak since knockout mice show markedly enhanced vessel leak leading to pulmonary oedema or death. Furthermore antibodies capable of inhibiting αvβ3 function caused dramatic increases in monolayer permeability in human pulmonary artery and umbilical vein endothelial cells in response to multiple growth factors. These data suggest a protective role for αvβ3 in the maintenance of vascular endothelial integrity following vessel stimulation and that inhibition of this function could drive pathogenic responses in a chronic disease setting (Su et al Absence of integrin αvβ3 enhances vascular leak in mice by inhibiting endothelial cortical actin formation Am J Respir Crit Care Med 2012 185: 58-66). Thus, selectivity for αvβ6 over αvβ3 may provide a safety advantage.
It is an object of the invention to provide αvβ6 antagonists.